In various continuous processes, it becomes desirable to measure the external dimensions of the objects being produced in a continuous format, and in this context various systems and apparatus have been devised to perform this duty.
The continuous processes with which the present invention is particularly concerned are extrusion processes for producing wire, cable and the like. However dimensional measurement may also be required in other processes such as those for glass tube or rod manufacture, or even in precision component manufacture.
The basic need to measure object dimension in these processes is to ensure that the product being manufactured conforms to specification, in terms of the eventual use to which it is to be applied where critical tolerances are essential.
A second reason is one of economy and this applies to the extrusion of plastic covered products where a minimum coating requirement has to be fulfilled to avoid excess coating going to waste.
Existing methods are based on a shadow graph or scanning principle, which either looks at the shadow of an object or scans the object and hence obtains a measurement of the overall size. These existing systems suffer from inaccuracies resulting from the object moving with respect to the measuring apparatus itself.
Object movement creates errors since this is interpreted falsely by the measuring instrument as a size change. This inherent difficulty has been overcome in some cases by guiding and restraining movement of the product through the measuring apparatus. This arrangement has had some but limited success.
Thus where the product is either too hot or too soft, the mechanical guidance system employed is liable to deform the product or render it useless.